To fill a content medium such as wine into a synthetic resin bottle, a so-called hot filling method, which involves filling of the content medium into a bottle at a temperature of, for example, from 50 to 70° C., followed immediately by capping to seal the bottle, may be used.
Since the bottle is sealed and subsequently cooled in the high filling method, the inside of the bottle is placed under a reduced pressure state. To absorb the reduced pressure, the bottle is known to be sometimes provided in the bottom thereof with a region (reduced pressure absorbing region) that is displaceable toward the inside of the bottle in response to the reduced pressure. (Refer to, for example, Patent Literature 1.)